The invention relates to a method for evacuating a low-vacuum chamber and a high-vacuum chamber of a high-vacuum apparatus, in which first the low-vacuum chamber and the high-vacuum chamber are evacuated together by means of pump trains each containing a vacuum pump unit and a vacuum forepump unit. The pump train of the high-vacuum chamber is used as a forepump for a high-vacuum pump unit of the high-vacuum chamber when the vacuum falls below a preset level. Furthermore, the invention relates to a high-vacuum apparatus for the practice of such a method.
In vacuum technology it is often necessary to evacuate two or more chambers in a vacuum apparatus to different working pressures. In vacuum depositing apparatus for coating substrates in band form, a pressure of less than 5.times.10.sup.-4 mbar must prevail, for example, in the chamber in which the vapor depositing process takes place, while in the winding chamber in which the substrate is wound and unwound a pressure of 5.times.10.sup.-2 mbar suffices.
To produce the necessary vacuums in such an apparatus, it has up to now been the practice to associate one pump train with the low-vacuum chamber configured as the winding chamber and another with the one serving as the coating chamber. When the changeover pressure is reached, the pump train of the high-vacuum chamber is cut off from the latter by means of a valve and used as the forepump for a high-vacuum pump unit then to be connected to the high-vacuum chamber. To assure a rapid draw-down and reliable operation of the high-vacuum pump unit, it is necessary to select the lowest possible changeover pressure. Especially in the case of dirty production apparatus this requires the pump train of the high-vacuum chamber to have a very great suction capacity in the pressure range between 1000 to 1 mbar. As soon as the pump train, however, is operating only as a forepump for the high-vacuum unit, and since then a preliminary vacuum pressure of 1.times.10.sup.-1 to 3.times.10.sup.-1 mbar is sufficient, all that is needed is only about one-tenth of the previously needed suction capacity. Therefore the pump train of the high-vacuum chamber is oversized for a far greater proportion of the time involved in the pumping cycle.